RhoA controls furrow formation

vidence that the cytokinetic apparatus is dependent on spindle microtubules comes primarily from classical experiments in which the spindle was manually moved or disrupted. On page 91, Bement et al. reveal the physiology that underlies these findings. Active RhoA localizes to a narrow zone at the site of future furrow formation in a microtubule-dependent manner, and the RhoA zone moves in response to spindle movement. RhoA, a GTPase that positively regulates actin–myosin contraction, has been detected before at the site of furrow formation. However , it wasn't clear if or how RhoA functioned in furrow formation, especially because both guanine nucleotide exchange factors, which activate RhoA, and GTPase-activating proteins, which inhibit are also in the furrow region. Bement et al. injected sea urchin and Xenopus embryos with a GFP reporter for RhoA activity. Using 4D imaging, they saw active RhoA accumulate in a tight band at the site of the future furrow. Nocodozole, which disrupts microtubules, prevented RhoA localization, but drugs that depolymerize actin did not. E Pushing the spindle upwards with a needle (dotted lines, top) moves the RhoA zone (white) upwards (bottom). A new mode of moving he well-described mechanism of movement by fibroblasts and other flat cells is characterized by outstretched lamellipodia, which are attached through focal adhesions, and retraction of a trailing edge. But how lym-phocytes move up to 100-times faster has been unclear. On page 141, Smith et al. report that T cells have a zone of clustered high-affinity LFA-1 integrin (␣ L ␤ 2) at the midzone of the cell, and that disruption of the pattern by removal of talin reduces the speed of cell migration. When T cells contact a monolayer of cells expressing ICAM-1, activated LFA-1 concentrates in the midzone of the cells, a region that the researchers referred to as the " focal zone. " The concentrated active LFA-1 in this focal zone colocal-ized with bound ICAM-1 in the supporting cells. Furthermore, the LFA-1 was unable to diffuse freely and was found to interact with the cytoskeleton via the T cytoskeletal linker protein talin. An siRNA knockdown of talin destabilized the LFA-1 focal zone and slowed T cell migration. By contrast, interference reflection microscopy showed that the lamellipodia A band of ICAM-1 (red) bound to the integrin LFA-1 helps T cells move quickly. made intermittent contacts with ICAM-1, but did not contain high-affinity, clustered LFA-1. The trailing edge uropod, which was …